(1) Technical Field
The present invention relates to an image forming apparatus by an electrophotographic system having a contact charging roll, and a method for producing a charging roll used therein. More particularly, it relates to an improvement of such an image forming apparatus that is capable of charging stably for a prolonged period of time, and a method for producing a charging roll used therein.
(2) Related Art
In recent years, a contact charging system is being used in an image forming apparatus, such as a duplicator, and in the contact charging system, a charging member applied with a charging voltage is made in contact with an image carrying member, such as a photoreceptor drum, to transfer charge directly to the photoreceptor drum, whereby the surface to be charged of the photoreceptor drum is charged to a prescribed potential. A member having a roll form is used as the charging member, and in general, has such a structure that prescribed functional layers, such as a conductive elastic layer, a resistance controlling layer and a releasing layer, are accumulated in this order around a conductive core metal.
The charging member in a roll form (hereinafter, which is referred to as a charging roll in some cases) charges a photoreceptor drum to a prescribed potential by utilizing discharge in a minute gap in the vicinity of the contact surface with the photoreceptor drum, and therefore, it has such a problem that the surface of the photoreceptor drum is liable to be abraded and deteriorated since the discharge in the vicinity of the contact surface strongly acts on the photoreceptor drum. In the case where the photoreceptor drum has a part with a film thickness that is not uniform, and the charging roll is close to or in contact with that part, there arises such a problem that localized wear occurs therein due to concentration of discharge. It has been known that these problems are liable to occur in both end parts of the charging roll, and defects, such as leakage, occur in those parts. This is because of the following factors. Discharge in the contact charging system occurs at a part that is slightly remote from the contact part between the photoreceptor drum and the charging roll. Accordingly, upon viewing the charging part of the conventional charging roll in the circumferential direction of the drum, the charging area in the both end parts of the charging roll is broadened in the circumferential direction of the drum since the end parts are left open, and thereby the charging time becomes longer in those parts than the other parts to accelerate deterioration of the photoreceptor drum.